Overshield
From Halopedia, the Halo wiki
The Overshield technology is used by the Covenant to enhance their Elite warriors' shields by a factor of three. It can be used by any Spartans that might come upon it. During multiplayer it is temporary and gradually deteriorates.
It looks simply like a red ball of energy encased in a glass cube (it is gold in Halo 2). A fan-theory states that once the user breaks the glass container, the energy dumps directly into the nearest energy field and increases it for a limited time. This theory hasn't been proven, because hardly any Covenant technology has been recovered intact by UNSC forces.
While present in both Halo: Combat Evolved's campaign and multiplayer, in Halo 2 it can only be found in multiplayer. It is likely that overshields are purely gameplay devices and have no actual presence in the Halo canon.
After walking over an overshield powerup, Master Chief's shield is raised from blue to red then green. In Halo 1's campaign, the overshield will not go down unless fired upon. You cannot pick up another overshield while still wearing one, no matter how low it is.
When wearing an overshield in Halo 2's multiplayer, the MJOLNIR armor is surrounded by a glowing force field, which is easily seen even if you are also wearing the Active Camouflage.
When picked up, it takes approximately three seconds to completely charge, but during the charging period the player will be invincible. When it is active, the normal shield does not take damage until the overshield is completely gone (although any damage overlapping from something destroying the overshield, for example a grenade's explosion, will take the full rest of its effect on the original shield/health). If the over shield is picked up when the shield is down, the player receives a full charge and the effects of the over shield stay. In the single player game, the overshield is reduced only when the player is hit, while in the multiplayer game, it also weakens at a constant rate (see MJOLNIR MARK V battle suit).
Note: Unlike in Halo 2, If you get an Over Shield and Active camouflage at the same time in Halo 1, you cannot be seen.
